It is said in “Margarine, Shortening and Lard” (Kimitoshi Nakazawa, Korin, Aug. 3, 1979, p 324) that “Margarine and shortening are defined to be plastic substances having crystalline fat at ambient temperature. Their physical properties relate chiefly to consistency, plasticity, and crystal structure. The crystal structures are physically classified into alpha (waxy, like acetoglycerides), beta (coarse crystals), and beta-prime (finely particulate). The melting point increases in the order of alpha, beta-prime, and beta. A preferred crystal structure for margarine and shortening is said to be beta-prime.” As so described, fats and oils of β-prime form have been accepted satisfactory and used for plastic fats and oils, such as margarine and shortening.
Fat and oil crystals of β-prime form are fine crystals which contribute emulsion stability to exhibit moderate consistency. On the other hand, because fat and oil crystals of β-prime form are energetically metastable, they are liable to transform to an energetically more stable β form under some conditions, such as improper storage conditions. Crystals of β form, the most stable form, do not transform further but generally have large crystal sizes and easily form coarse crystal grains, called graying or bloom, which give a rough and unpleasant texture and render the products commercially valueless.
There exist crystals of β form transformed from β-prime form and yet having a relatively small crystal size. Cocoa butter of V-form is among them. The V-form cocoa butter consists substantially of β-2 form crystals of symmetric triglycerides, such as SOS and POS. Production of such β form crystals of relatively small size involves an extremely complicated step, such as a special heat treatment called tempering or a step of cooling to a prescribed temperature followed by addition of a specific component as crystal nuclei. Under the present situation, it is impossible to obtain them through rapid cooling plasticization as carried out in the manufacture of ordinary fat and oil compositions. Moreover, cocoa butter of V-form lacks plasticity.
Even fats and oils of the most stable β-prime form have tendency to harden with time and need careful management on crystallization, preservation, and the like.
To address these problems, various inventions have so far been proposed in an attempt to obtain energetically stable and fine crystals. For example, JP-B-51-9763 discloses a process for obtaining β form crystals by specifying the ratio of triglycerides. JP-B-58-13128 teaches a process of suppressing graying of fats and oils by interesterification. JP-A-10-295271 proposes a process in which a high-melting fat and oil is incorporated to maintain the fine crystal form.
The process of JP-B-51-9763 involves tempering for obtaining β form crystals. The compositions obtained by the processes of JP-B-58-13128 and JP-A-10-295271 tend to harden with time and cannot be seen as sufficiently satisfactory in stability as a fat and oil composition. It has turned out that, when the oil phase of a fat and oil composition prepared by any of the disclosed processes is completely melted at 70° C. and maintained at 0° C. for 30 minutes and then at 5° C. for 7 days, the resulting fat and oil crystals are not β form crystals of double chain length structure but β-prime crystals of double chain length structure or β crystals of triple chain length structure.
Although a so-called “compound type” roll-in fat and oil composition which contains milk fat provides a satisfactory flavor, it has a narrow temperature range for plasticity, tending to harden in low temperature and be cut during roll-in, and is not sufficiently satisfactory from the aspect of workability.